The male golden hamster is an important model in which to study the integration of sensory and hormonal signals which control reproductive behavior. In the brain of this species the essential chemosensory pathways and the target sites for testosterone (T), and its metabolites estradiol (E) and dihydrotestosterone (DHT) have been mapped. Neurons in three nuclear areas along these pathways - the medial amygdaloid nucleus (M), the medial bed nucleus of the stria terminalis (BNSTm) and the medial preoptic area (MPOA) - actively accumulate gonadal hormones. The neural connections between these three regions and the influence of gonadal hormones on their neurons are the continuing subject of this project. We have found that T maintains the dendritic arborization of neurons in M and the Substance P production of neurons in M, BNSTm and MPOA. In this proposal, neuronal connections from the rostral to the caudal parts of M are hypothesized to connect regions of this nucleus which differ in their connections, hormone dependence, and behavioral significance. These projections will be studied after iontophoresis of the orthogradely transported tracer Phaseolus vulgaris leucoagglutinin into rostral M and, in other experiments, iontophoresis of the retrogradely transported tracer fluorogold into caudal M. Second, projections made by the hormone-concentrating neurons of M, BNSTm and MPOA will be investigated by combining the injection of retrograde fluorescent tracers (e.g., primuline) with steroid autoradiography. This will double-label hormone-accumulating cells which make specific neuronal connections. Third, the connections of those neurons in M, BNSTm and MPOA which produce Substance P will be studied by immunohistochemical identification of Substance P cells. Last, some specific functions of T and its metabolites in this pathway will be investigated by replacing either T, E, DHT or DHT plus E systemically in castrated males, and comparing the effectiveness of these treatments in maintaining normal dendritic morphology (measured in Golgi preparations) and Substance P production (measured immunohistochemically). Taken together these experiments will further define the neurotransmitter production and hormonal dependence of neurons comprising the mating behavior pathway.